1. Field of the Invention
The invention relates to an image forming apparatus capable of forming images on both sides of a copy sheet.
2. Background
In conventional image forming apparatuses capable of forming images on both sides of a copy sheet, when making a plurality of duplex copies, a plurality of copy sheets accommodated in a paper cassette are sequentially fed therefrom and images are formed on the first side of said sheets, and the sheet is subsequently inverted front-to-back and temporarily stored in a re-feeding tray (hereinafter referred to as "intermediate tray") within the apparatus. When image formation on the first side of said plurality of sheets is completed, the copy sheets stored in the intermediate tray are re-fed therefrom, and images are formed on the second side of said sheets, whereupon the sheets are ejected to a discharge tray.
On the other hand, copying machines have been proposed which manage the number of copies that can be made by presetting a maximum number of copies permitted for specific users, and prohibiting a copy operation executing when the accumulated number of copies made by said specific user exceeds said maximum copy number. For example, it is possible to prerecord information on an identification (ID) card including an identification number for a department, a maximum number of copies permitted for said department, and the number of accumulated copies executed by said department, such that copying machines installed in this company can read the information contained by the ID card when the copying machine is used, and prevent a copy operation when the number of accumulated copies reaches the maximum number of permitted copies.
When making multiple sheets of duplex copies in the aforesaid copy machine which has a set maximum copy number limitation, however, a copy operation may be stopped when the maximum copy number is attained during the execution of said duplex copies on multiple sheets, such that some copy sheets may only have images formed on a first side and remain in the intermediate tray.
When desiring to continue duplex copying in such cases, it is necessary to switch the ID card for another indicating the maximum number of copies has not been attained, and then restart the copy operation. Normally, when there is no key input for a predetermined time interval, a copying machine automatically resets the copy mode previously set by the operation panel to improve operation characteristics, or automatically turns OFF the power source to conserve power. Accordingly, when a copy operation is stopped because the number of accumulated copies reaches the set maximum copy number, the copy mode may be reset or the power source may be turned OFF when the aforesaid predetermined time interval elapses before the ID cards can be switched because a new ID card is not readily available. Furthermore, the copy mode may be reset or the power source may be turned OFF when an operator temporarily leaves the site of the copy machine and is unaware that a copy operation has stopped. The copy mode also may be reset when an ID card is pulled from the card reader to replace the ID card.
In the above cases, of course, since a duplex copy mode will also be reset, copy sheets accommodated in the intermediate tray which have not undergone image formation on the second side will have to be removed from said intermediate tray by the operator and reset in the feed tray while taking care to maintain the sheet direction and front-to-back orientation in order to complete the unfinished copying on the second side of said sheets.
Since this procedure is unusually difficult and has a severe adverse affect on operating efficiency, it is typical that the copy sheets in the duplex tray are wastefully disposed of and the duplex copy operation is started over again from the beginning.
The above-described disadvantages are particularly inconvenient in the case of so-called coin vending type copying machines used in convenience stores and the like wherein coins are inserted to purchase an exact number of copies, inasmuch as a copy operation may be stopped during processing when the specified number of duplex copies exceeds that purchased by the inserted coins, thereby preventing the user from achieving the duplex copy objective if sufficient coins are not available.